Exposure of precontracted vascular tissues to anoxiain vitrocauses a rapid but short lasting additional contraction of vascular smooth muscle. To determine the effect of sustained anoxia on vascular reactivity, quiescent rat aortic rings were equilibrated at 37°C under 95% O2(normoxia) or 95% N2(anoxia) for 15 min. The rings were then contracted with norepinephrine (NE) or the TxA2-endoperoxide analog 1146,619. The response of aortic rings to the endothelium-dependent vasorelaxant stimuli acetylcholine (ACh) and calcium ionophore A23,187, the endothelium-independent stimuli nitroglycerin (NTG) and sodium nitroprusside (SNP) and the nitric oxide-independent stimulus isoproterenol (ISO) was also examined. Vascular contraction was markedly decreased in anoxic rings (P<0.01versusnormoxic rings) in response to both NE and U46.619 and was not modified on treatment with indomethacin, L-NMMA or Nω-L-arginine. Relaxation of anoxic rings was also markedly impaired (P<0.01) in response to the endothelium-dependent vasorelaxants ACh and calcium ionophore A23,187. Rings treated with ACh showed much less cyclic GMP accumulation during anoxia than during normoxia (P<0.02), indicating decreased EDRF synthesis during anoxia. The degree and rate of relaxation of anoxic rings in response to the endothelium-independent vasorelaxants NTG and SNP were also markedly diminished (P<0.01versusnormoxic rings) although cyclic GMP accumulation in anoxic rings treated with NTG was the same as in normoxic rings, suggesting diminished tissue sensitivity to cyclic GMP. Relaxation in response to ISO in anoxic rings was also markedly decreased. These observations indicate that reduced endogenous EDRP synthesis and tissue sensitivity to cyclic GMP and cyclic AMP-dependent relaxants are involved in the anoxia-associated decrease in vasorelaxation.